1. Field of Invention
The present invention relates to a device and method for optimally detecting a surface condition of different types of wafers by automatically selecting filters to appropriately cut off a certain amount of reflected ray from impinging on an imaging sensor to process wafers with different surface properties.
2. Discussion of Related Art
FIG. 1 shows elements of a conventional device for detecting the surface condition of a wafer. Referring to FIG. 1, the conventional device comprises an Xe-lamp 1 for emitting a constant emissive ray, a filtering lens 3 for filtering the emissive ray from the Xe-lamp 1 within a band of 400.about.700 nm, a reticle 5 for adjusting the focus of the emissive ray transmitted through the filtering lens 3, a half mirror 7 for directing the filtered ray to a beam splitter 17 and to a mirror 8, the beam splitter 17 for splitting the ray reflected from the half mirror 7 to each PCD (Position Charge Device) sensor 13 and 15, the PCD sensors 13 and 15 determining the optimal points of automatic focus for the ray, amplifiers 19 and 21 for respectively amplifying signals output from the PCD sensors 13 and 15, an image processing device 23 for comparing the signals output from the amplifiers 19 and 21 to each other to determine the best optimal point of automatic focus for the wafer 27, a piezo driving device 25 for controlling a piezo member 29 to provide the best optimal focus point for the wafer 27 according to a signal output from the image processing device 23, and the piezo driving device 25 for controlling the piezo member 29. The piezo member 29 is placed on a side of the stage on which the wafer 27 is placed for three-dimensionally controlling the position of the stage.
The conventional device further comprises the mirror 8, an objective lens 9 for magnifying a ray reflected from a wafer 27 and directing it to the mirror 8, and a CCD (Charge Coupled Device) sensor 11 for representing surface condition of the wafer 27 based on the reflected ray output from the objective lens 9. The output of the CCD sensor 11 is processed by the image processing device 23 to determine any abnormality on the surface of the wafer 27.
An operation of the conventional device of FIG. 1 is as follows.
Referring to FIG. 1, an emissive ray is projected from the Xe-lamp 1 in the direction of the stage where the wafer 27 is placed. The emissive ray is filtered by the lens 3 to a wave band of 400.about.700 nm suitable for detecting particles. The filtered ray is reflected by the half mirror 7. The beam splitter 17 splits the reflected ray into two beams which are respectively directed to two PCD sensors 13 and 15. The amplifiers 19 and 21 respectively amplify the outputs of the PCD sensors 13 and 15.
The image processing device 23 determines the best optimal point of automatic focus for the wafer 27 by finding the intersection position of electrical signals output from the two PCD sensors 13 and 15, and generates a control signal to the piezo driving device 25. The piezo driving device 25 controls the piezo member 29 based on the control signal so that the Xe-lamp 1 projects light on the wafer 27 at the optimal point of automatic focus. That is, in accordance with a driving signal output from the piezo driving device 25, the piezo member 29 displaced at one side of the stage having the wafer 29 thereon is moved to achieve the optimal point of automatic focus for the light impinging on the wafer 27.
At the same time, the ray emitted from the Xe-lamp 1 and reflected from the wafer 27 passes through the objective lens 9 and is reflected by the mirror 8. Then this ray impinges on the CCD sensor 11, and is visualized subsequently to identify the pattern of the surface of the wafer 27. The CCD sensor 11 generates digital signals representing any abnormality (e.g., particles, dents, etc.) on the surface of the wafer 27. The principle of the CCD sensor 11 is to detect any displacement of the wafer surface by reading and analyzing light-receiving spots (photo-diode) of the reflected ray. The image processing device 23 processes the output of the CCD sensor 11 to detect an abnormality on the surface of the wafer 27.
In the conventional device of FIG. 1, the emissive ray from the Xe-lamp 1 is reflected at the surface of the wafer 27, and then transferred to the CCD sensor 11 so that the wafer surface can be examined. However, since the reflection rate at the surface of each wafer differs from each other, wafer abnormality will be detected only if the reflection rate of the wafer is high. Otherwise, surface abnormality of the wafer can not be detected effectively since the processing of a weak reflective ray by the CCD sensor can result in inaccurate detection results. Therefore, conventional detection devices and methods are not reliable when detecting the surface condition of different types of wafers.